Sara Gilbert's elder sister was a big TV star, Melissa Gilbert of Little House on the Prairie, so Sara was naturally drawn to acting, just to compete. At nine, she made her professional debut in a TV movie of Calamity Jane, and at 13 she won the role of Darlene, the troublesome younger daughter on Roseanne. She played the role for ten years, matching the span of Melissa's run with Little House. Gilbert also wrote one of Roseanne's better episodes, "Don't Make Me Over", in which Roseanne receives a special birthday makeover from her daughters, but learns that they had an ulterior motive.

Since Roseanne, Gilbert's subsequent TV sitcoms have included Welcome to New York with Christine Baranski, and Twins with Melanie Griffith. On the big screen, she kissed Drew Barrymore in Poison Ivy, seized control of her high school in Light It Up, and in the film Thirty Bucks she played a prostitute available for that price. In John Cusack's High Fidelity, she had a brief bit as a hippie girl.

Gilbert is an ex-smoker and an out lesbian, and very active politically, volunteering with People for the Ethical Treatment of Animals, the Pediatric AIDS Foundation, Meals on Wheels, and AIDS Project Los Angeles.